Akershus Castle and Fortress

Sitting in the center of Oslo, the Akershus Castle and Fortress has been in continuous use for more than 700 years. For most of that time, its primary purpose was to defend the city from foreign invaders -- something it did well, as no foreign military ever managed to capture it by force. The fortress also served as a prison for a time.

One of the site's major highlights is the Castle Church. You can even attend services here during the spring (mid-March to June) and autumn (August until the first Sunday in Advent Sunday). Worship takes place on Sundays at 11 a.m.

You can tour Akershus Castle and Fortress' grounds for free, but entering the castle itself will cost you around $11 USD (65 kroner) unless you have an Oslo Pass, which includes admission to the castle, as well as spots like the Kon-Tiki and Munch museums. For a complete listing of places that you can get into with the Oslo Pass, click here.

The grounds are open from 6 a.m. until 9 p.m.; from May 2 to Aug. 31, you can enter the castle anytime from 10 a.m. until 4 p.m. On Sundays the site opens at 12:30 p.m.  From Sept. 1 to April 30, your only chance to see the castle is on one of the Thursday guided tours. The only English tour is at 1 p.m. and it lasts around 50 minutes. For more information, click here.